This project will investigate the cellular basis of vocal learning in songbirds. Behavioral studies have established that male songbirds learn their songs from their fathers, and investigations using anatomical and electrophysiological approaches have elucidated the brain circuitry necessary for vocal learning. Work in a variety of systems has pointed toward a critical role for synaptic plasticity in learning. It is now possible to make and test specific cellular hypotheses concerning how changes in synaptic strength could give rise to vocal learning in songbirds, but little is known of the cellular electrophysiological properties of the neurons in the songbird brain. The proposed experiments will investigate neurons in nucleus RA of the songbird brain, an important site of convergence of auditory and motor information which has been implicated in song learning. Voltage-clamp recordings will be obtained from neurons in in vitro brain slices to test the hypothesis that activity-driven synaptic plasticity, resembling long- term potentiation (LTP) or depression (LTD) as studied in other preparations, could contribute to song learning. Previous findings indicate that the auditory and motor pathways projecting to RA neurons use different combinations of postsynaptic neurotransmitter receptors. The existence of synaptic plasticity in one or both of these pathways will be tested by applying, individually and in combination, various types of stimuli that are effective in other systems exhibiting plasticity. If a form of synaptic plasticity is found, it will be necessary to test whether it might play a role in behavioral plasticity. Although it is difficult to design experiments to test this hypothesis directly, a number of indirect tests are possible. The developmental profile of synaptic plasticity will be investigated to determine whether changes in synaptic strength become less pronounced or more difficult to induce at times when the bird's song is not plastic. Secondly, by shifting the critical period for song learning using hormonal manipulations, it should be possible to shift the critical periods for cellular changes. These studies will be among the first to examine, in a vertebrate system, the link between changes in synaptic strength and learning of a complex behavior.